I am working on a small synth and I wanted to have one knob that detunes one oscillator from the main note.If someone detunes it and then wants to put it back to where it was, on a normal potentiometer you will have to use your ear.So I was thinking of having a potentiometer/knob like the one on some radios where you control the balance: When you reach the normal (center) position it clicks and goes to the exact middle, if you want to move it from the center the knob resists a little.

Could I buy it somewhere? What's called?

BONUS: Also what about ribbon resistor? A band that gets different R values depending on where you press it.

Center detent would be the simplest and do exactly what you want. Another option may be using a rotary encoder, perhaps with is push button built in. Push the knob, back to center. The advantage of a rotary encoder would also be that you could give it an auto snap back sort of thing. Say you press it while turning, when you let go it goes back to center on its own with possibly even a specific transition speed.